1. Field of the Invention
The present invention relates to a line sensor camera that shoots a two dimensional image by means of a line sensor (a one dimensional image pickup device).
2. Description of the Related Art
A digital camera that effects shooting using a combination of a line sensor and a scanning mechanism such as a mirror rotation mechanism has been conventionally proposed (Japanese Patent Publication No. 4-67836). This type of digital camera is called a line sensor camera. A line sensor camera has the advantage of allowing shooting at a resolution higher than that of a digital camera that employs an area sensor. The entire page of a document of approximately A4 in size, for example, can be photographed at a level where regular-sized characters on the page can be read.
In a line sensor camera, the scanning rate on the plane of the object to be taken varies depending on the image pickup angular position when the scanning mechanism rotates uniformly. Referring to FIG. 18, the image pickup distance d2 per unit time at the edge area of object plane S1 is greater than the image pickup distance d1 per unit time at the center area. The photo image will result in a distorted image where the edge portion of the object is compressed.
As conventional methods to eliminate such image distortion inherent to rotation scanning, the method of scanning object plane S1 at uniform speed optically using a f.theta. lens, and the method of varying the rotary speed of the motor for scanning according to the image pickup angular position of secondary scanning (Japanese Patent Laying-Open No. 4-203915) are known. The former is not applicable to a handy-type camera since it is difficult to fabricate a f.theta. lens that is small, light, and bright. Furthermore, in the structure where the light of the object always passes through the optical axis (center) of the lens, correction using the f.theta. lens cannot be effected since the .theta. does not change.
In such a line sensor camera, the degree of freedom in framing is increased to improve user's convenience by incorporating a zoom unit as the lens system for projection. However, in a line sensor camera of a structure in which the optical axis of the lens system moves relative to the object to be photographed in conjunction with secondary scanning such as a line sensor camera in which the mirror for secondary scanning is located in front of the lens system, there is a problem that the aspect ratio of the object image (photo image) on the image pickup surface of the line sensor is altered when the focal length of the lens system changes according to a zooming operation, i.e. when the image magnification changes. It was necessary to alter the image pickup range of the secondary scanning direction according to the focal length. Furthermore, there is a case where the image pickup range in the secondary scanning direction, not the main scanning direction, is altered to achieve the effect of panoramatic shooting. In such cases where the image pickup range has to be altered, the scanning rate must be increased to restrict the image pickup time period for the purpose of reducing the effect of camera-shaking when the image pickup range is expanded. In other words, the scanning rate must be controlled in response to change in the image pickup range.
The aforementioned method of employing a f.theta. lens is not applicable to a portable camera. Fabrication of a clear f.theta. lens that is small in size and weight and that allows zooming is extremely difficult. In the case of a structure where the mirror for scanning is arranged in front of the lens, the f.theta. lens cannot be used since the .theta. cannot be altered due to the light of the object always passing through the optical axis (center) of the lens.
The method of changing the rotary speed of the motor is disadvantageous in that the rotary speed cannot be controlled critically in comparison to the control where the rotary speed is maintained at a constant level during shooting.